1. Field of the Invention
The present invention relates to an internal combustion engine fitted with a sensor arranged to detect knocking. The present invention also relates to a straddle-type vehicle equipped with the engine.
2. Description of the Related Art
An internal combustion engine can cause knocking in some cases depending on its operating conditions. Knocking should be avoided as much as possible because it results in, for example, unusual noise and performance degradation of the internal combustion engine. Conventionally, it is known that a sensor to detect knocking, that is, a knock sensor, is fitted to an internal combustion engine. It is also known that, upon detecting knocking by the knock sensor, an action such as changing ignition timing is taken.
JP 2004-301106 A discloses a water-cooled engine in which a knock sensor is fitted to a cylinder block.
A water-cooled engine needs a flow passage for coolant, i.e., a water jacket, to be provided in, for example, a cylinder block and a cylinder head. It also requires, for example, a pump to convey the coolant and a radiator to cool the coolant. For this reason, the structure of the water-cooled engine tends to be complicated.
A straddle-type vehicle equipped with a single-cylinder internal combustion engine (hereinafter referred to as a “single-cylinder engine”) is known, such as a relatively small-sized motorcycle. The single-cylinder engine has the advantage that it has a simpler structure than a multi-cylinder engine. To fully exploit the advantage, the single-cylinder engine has a relatively simple cooling structure. For that reason, conventionally, at least a portion of the cylinder block and the cylinder head is cooled by air.
In a single-cylinder engine at least a portion of which is cooled by air, temperature variations may occur locally depending on the flow of the air around the engine. In other words, depending on the flow of the air, there may be a portion in which the temperature is high locally and a portion in which the temperature is low locally. If the knock sensor is mounted to the portion of the engine in which the temperature is high, the knock sensor is heated by the engine, and the temperature of the knock sensor is raised excessively. As a consequence, the reliability of the knock sensor may become lower.
In a water-cooled type engine as well, temperature variations may be caused locally depending on the shape or dimensions of the water jacket, or the flowing condition of the coolant, and the like. Accordingly, the same problem as described above may arise.